In the manufacture of electrical equipment, electrical insulation is provided in the form of glass fabric, thermoplastic film, and mica paper or mica flake tapes which are impregnated with curable polymeric materials. The fabric, film, paper or tape or other suitable form is either treated with polymeric material before application to a conductor, i.e., pre-impregnated, or afterward as by the vacuum-pressure impregnation technique employed in the production of coils for motors and generators. In either case, the resin composition must be applied and cured in place without voids which reduce useful life of the insulation as a result of breakdown under electrical stress. For this reason, the resin composition must be effectively solvent free and at that same time it must be of relatively low viscosity for easy flow around and between the laps of insulating tape of a coil and for similarly efficient penetration in the preparation of pre-impregnated materials. While relatively low viscosities are less critical for preparing pre-impregnated insulation because the impregnation temperature can be increased to decrease viscosity, the tendency towards reaction or gelation at such elevated temperatures precludes the use of some otherwise desirable high viscosity compositions from use for pre-impregnated insulation.
Epoxy resins are usually preferred to polyester resins because of their substantially superior characteristics of thermal stability, adhesion, tensile, flexural and compressive strengths, resistance to solvents, oils, acids and alkalis. However, the viscosity of these resins typically run the order of 4,000 to 6,000 centipoises (cps) and worse, when certain hardeners are added, their viscosities run to the range of 7,000 to 20,000 cps which is much too high for useful impregnation purposes. While viscosity of that sort can be reduced substantially through the use of certain epoxy diluents, attempts along this route in the past have only served to decrease the thermal stability of the compositions and thus result detrimentally to the electrical properties of the cured insulation.